Direct contact evaporators

ABSTRACT

A direct contact evaporator for concentrating residual pulp liquor where the concentration of the liquor is accomplished by hot air. The combustion of the concentrated liquor in a boiler produces hot gases which are used to produce steam and to preheat the feed water with steam and hot water utilized in indirect heat exchangers to heat the air used in concentrating the liquor. Additional heat is applied to the air as generated by separate fuel combustion in a separate indirect air heater.

ilnited States Patent Pacault et a1.

1451 Dec. 25, 1973 DIRECT CONTACT EVAPORATORS FOREIGN PATENTS OR APPLICATIONS [75] Inventors: Pierre Henri Pacault, Ville dAvray; 674,533 6/1952 Great Britain-m I 2 Marc Kagans, Asnieres both of 694,552 9/1964 Canada 122 1310. 2 France OTHER PUBLICATIONS Assigneel The Babcock & Wilcox p y, Babcock & Wilcox Co., Bulletin P-2, Copyright 1959,

New York, NY. pp. 9, l5, l7, 19 [22] Filed: Jan. 21, 1971 Primary Examiner-Norman Yudkofi [21] Appl' 108353 Assistant ExaminerJ. Sufer Attorney-J. Maguire [30 I Foreign Application Priority Data Jan. 29, 1970 France 7,002,059 [57] ABSTRACT A direct contact evaporator for concentrating residual 52 11.8. C1. 159/4 A, 159/16, 122/D1G. 2 p p liquor Where the concentration of the liquor is [51] Int. Cl B0111 1/16 accomplished y hot The combustion of the [58] Field of Search 159/4 A, 47 WL; cehtrated liquor in a boiler Produces hot gases which 2/30; 23/4 273 are used to produce steam and to preheat the feed water with steam and hot water utilized in indirect [56] Ref nces Ci d heat exchangers to heat theair used in concentrating the liquor. heat 18 t0 the air as gen- 1 975 519 10/1934 R d f erated by separate fuel combustion in a separate indiu or 2,593,503 4 1952 Tomlinson et a1. 23/48 met heater 2,635,587 4/1953 Dalin et al. 122/1 I 2,699,758 l/l955 Dalin I 122/1 A 3 Cl m 1 Drawing Figure 2,707,458 5/1955 Hayden... 122/1 A 2,713,330 7/1955 Taylor 122/1 A 2,762,345 9/1956 Bayard.......................,.... 122/1 A 3,574,051 4/1971 Shah 162/30 34 l I 32 I 26 I V HOT AH A.H AIR FAN 40 I 8 30 24 I l BURNER J l l -l 1-"--.''

l l H o VAPOR 2 I 1 A 4\ 28 e sTAcK 1 2";

i I l- 'l I 6 ousr l l i 1 EXTRACTOR l 1 9 I, U /'\J LIQUOR l 1 2 1 ECONOMIZER IN 1 I l l J 1 H O V RATOR 14 1 com/311N101, i EVAPO LQUOR CHAMBER 3 our GAS BY-PASS DIRECT CONTACT EVAPORATORS This invention relates to direct contact evaporator units in which the calorific content of a liquid is utilised by burning the liquid in a combustion chamber following concentration of the liquid in a direct contact evaporator utilising a portion of the heat released in the combustion chamber to effect evaporation of the liquid. More particularly, the invention is concerned with the combustion of black liquor produced in papermaking plant.

In the paper-making and cellulose industry, the boiling treatment of wood, cut up into small pieces, in a lye containing chemical products in a cooker supplies cellulose on the one hand and, on the other, black liquor charged with lignin and chemical products. This liquor is fed to a waste heat boiler to recover the thermal energy and mineral material. Its dryness, however, is only (for example) percent at the outlet of the cooker and must be increased before discharge at the burners to a value of between about 55 and 70 percent, according to the qualities of the liquor and the method of combustion adopted.

The liquor is initially concentrated in multi-acting evaporators which use steam as heating medium. The further the evaporation is extended, the greater are the risks of fouling of these evaporators. The dryness is therefore merely brought to a value of between about 45 and SOpercent in the steam heated evaporators and further concentration is effected in direct contact evaporators, from which the liquor is discharged with the required dryness.

In such direct contact evaporators, which are of various types, for example, cyclone, Venturi or cascade, the black liquor is directly contacted by a hot fluid, usually the exhaust gases discharged at the economiser of the waste heat boiler. The disadvantage of this is that the exhaust gases discharged into the atmosphere after passing through the direct contact evaporator carry volatile material from the liquor together with combustible products, mercaptans and other badly smelling, sulphurous compounds, which pollutes the atmosphere and also reduces the recovery of the calorific content of the liquor as represented by such products and compounds.

Whilst it has previously been proposed to use air as heating medium in the direct contact evaporator, the air being first heated, by means of the combustion gases, in an indirect surface heat exchanger, such indirect, surface heat exchangers are particularly vulnerable to fouling, and the cleaning operation which then becomes necessary is liable to disturb the operation of dust-extracting electrostatic filters arranged downstream.

A direct contact evaporator unit according to the present invention includes a direct contact heat exchanger arranged to concentrate a liquid having a volatile content, a combustion chamber provided with fluid heating surfaces and air heating means arranged to transfer heat to air prior to discharge to the direct contact heat exchanger from the fluid heated in the combustion chamber.

The invention will now be described, by way of example, with reference to the accompanying diagram.

in the diagram, the combustion gases from a black liquor fired steam generating combustion chamber 2 pass successively, through an economiser 4 and a dust extractor 6, before being discharged through a chimney 8. The economiser 4 is preferably dimensioned to absorb all the sensible heat that the combustion gases are capable of yielding in practice by surface heat exchange. Thus, in one example, the combustion gases are discharged from the economiser at a temperature of about 150C.

A direct contact evaporator 10 arranged to effect concentration of black liquor, the calorific content of which it is desired to utilise by direct contact heat exchange between the liquor and hot air. Liquor is supplied to the evaporator 10 through an inlet 12 and is discharged, through an outlet 14, to a burner 16 in the combustion chamber 2 whilst hot air flows, in opposition to the liquor flow, from an inlet 18 to an outlet 20 discharging to the unit.

Prior to discharge to the evaporator 10, the air is heated during passage through three air heaters, 26, 32 and 34, arranged in series in a hot air supply path represented by abroken line, connecting the direct evaporator 10 and a forced draft fan 38. I

The air heaters 26 and 32 respectively utilise as heating fluid hot water and steam discharged through pipes 24 and 30 respectively from the economiser 4 and the combustion chamber 2 as supplied through feed water connections 22 and 28, while the air heater 34 is fired with an auxiliary supply of gas or liquid fuel delivered through line 36.

A valved by-pass 40 is provided around the fired air heater 34 and the evaporator 10 to permit the supply of heated air direct to the combustion chamber 2 when the black liquor is supplied to the evaporator at a reduced rate, to avoid undue concentration thereof.

In operation, the black liquor is concentrated in the evaporator 10 by direct contact with the hot air to a degree of dryness required to enable it to be admitted to the burner 16 as liquid fuel. The combustible volatile products carried over in the hot air from the evaporator 10 are burnt in the combustion chamber 2 thereby enhancing the utilisation of the calorific content of the liquor. Further, this also avoids discharge to the atmosphere of malodorous or harmful combustible substances which are separated from the solution by direct evaporation, either completely or substantially.

The sensible heat of the combustion gases is thus utilised to the fullest extent practicable in the evaporator 10 through the medium of the steam generating tubes of the combustion chamber 2 and economizer 4, which do not suffer from the fouling difficulties to which the tube surfaces of combustion gas heated air heaters are subject. Since the combustion gases are not discharged direct to the evaporator 10, the temperature of the gases at the outlet from the economiser 4 may be lowered to give a high overall steam efficiency.

According to an alternative arrangement (not 7 shown), a further indirect, evaporator air heater is provided utilising as the heating medium the combustion gases from the chamber 2 before entry into the economiser 4.

In an another alternative arrangement the hot air is recycled through a valved duct from the hot air-vapor outlet 20 of the evaporator 10 to the inlet of the fan 38. This arrangement enables the mass of air in circulation in the evaporator 10 to be increased, without increasing the flow of hot air to the boiler, and consequently enables the amount of heat input at the indirect heat exchangers 26, 32, 34 to raise the air temperature to that necessary for the concentration of the liquor, to be reduced.

Whilst the foregoing description is concerned with the combustion of black liquor, it will be appreciated that the arrangement is equally applicable to any other installations which include a direct hot air operated evaporator using as a source of heat combustion gases of an associated combustion chamber.

We claim:

1. In apparatus for the recovery of chemical and heat from pulp residual liquor containing inorganic chemicals and combustible organic matter comprising a boiler including a furnace for the combustion of concentrated residual liquor therein and an economizer, said furnace having vapor generating tubes disposed therein operable to heat liquid therein from the hot gases produced during combustion of the organic matter in the residual liquor, vaporizable liquid heating tubes positioned in said economizer to absorb heat from the combustion gases and to deliver the heated liquid from the economizer to the vapor generating tubes of said furnace, the improvement comprising a direct contact evaporator for concentration of residual liquor by evaporation of water therefrom in direct contact with hot air, the evaporator having an inlet for hot air and an outlet for cooled air, an inlet for liquor and an outlet for concentrated liquor connected with the furnace, a duct connecting the cooled air outlet from said direct contact evaporator with the furnace, and air heaters arranged for sequential flow of air therethrough, wherein one indirect contact air heater is supplied with heated liquid from the economizer, another indirect contact air heater is supplied with vapor from said boiler and a separate indirect fuel fired airheater is provided to effect additional heating of the air prior to discharge to the direct contact evaporator.

2. In the apparatus of claim 1, wherein the separate fuel fired air heater is positioned to receive air from the air heaters and discharge to the direct contact evaporator and a valved by-pass line is provided around the fuel fired air heater and the direct contact evaporator to discharge the heated air from the air heaters direct to the furnace.

3. In the apparatus of claim 2, wherein a valved duct is provided between the air-vapor mixture outlet from the direct contact evaporator and the air inlet to a forced draft fan the exhaust of which leads to the air heaters to permit recycling of air-vapor mixture through the direct contact evaporator. 

1. In apparatus for the recovery of chemical and heat from pulp residual liquor containing inorganic chemicals and combustible organic matter comprising a boiler including a furnace for the combustion of concentrated residual liquor therein and an economizer, said furnace having vapor generating tubes disposed therein operable to heat liquid therein from the hot gases produced during combustion of the organic matter in the residual liquor, vaporizable liquid heating tubes positioned in said economizer to absorb heat from the combustion gases and to deliver the heated liquid from the economizer to the vapor generating tubes of said furnace, the improvement comprising a direct contact evaporator for concentration of residual liquor by evaporation of water therefrom in direct contact with hot air, the evaporator having an inlet for hot air and an outlet for cooled air, an inlet for liquor and an outlet for concentrated liquor connected with the furnace, a duct connecting the cooled air outlet from said direct contact evaporator with the furnace, and air heaters arranged for sequential flow of air therethrough, wherein one indirect contact air heater is supplied with heated liquid from the economizer, another indirect contact air heater is supplied with vapor from said boiler and a separate indirect fuel fired air heater is provided to effect additional heating of the air prior to discharge to the direct contact evaporator.
 2. In the apparatus of claim 1, wherein the separate fuel fired air heater is positioned to receive air from the air heaters and discharge to the direct contact evaporator and a valved by-pass line is provided around the fuel fired air heater and the direct contact evaporator to discharge the heated air from the air heaters direct to the furnace.
 3. In the apparatus of claim 2, wherein a valved duct is provided between the air-vapor mixture outlet from the direct contact evaporator and the air inlet to a forced draft fan the exhaust of which leads to the air heaters to permit recycling of air-vapor mixture through the direct contact evaporator. 